The Prime Minister was warned that fewer seizures of the drugs used to make methamphetamine were a sign smugglers had changed their methods - rather than less drugs coming into the country as previously stated by a Cabinet colleague.
The warning came shortly before a record haul of pseudoephedrine was discovered in Auckland.
Operation Ghost found 330kg hidden in three safe houses and detectives believe another 142kg was imported in May. Intercepted conversations led to another 248kg being stopped at the port.
Seizures of pseudoephedrine, which is a legal cold medicine in China, had been steadily falling over the past four years since government measures to curb the P trade.
The Customs Minister, Maurice Williamson, said 300kg of precursor drugs had been found in 2012 - down from a record 1.2 tonnes in 2009.
"We believe measures the Government has taken since 2009 have resulted in the supply of precursors being constrained and it is reflected in the statistics," he said.
But a working group set up by Prime Minister John Key was warned that although smaller amounts of pseudoephedrine were being stopped, this did not necessarily mean less was being smuggled in.
"The ongoing decline in the quantity of precursors seized is likely to be a reflection of a change in modus operandi by the syndicates involved, rather than an indication of reduced quantities entering New Zealand," according to October's Tackling Methamphetamine report from the Department of the Prime Minister.
Hundreds of kilograms of pseudoephedrine seized in Operation Ghost were disguised as bags of breadcrumbs or starch shipped from China and destined for a yum cha restaurant in downtown Auckland.
A spokesman for Mr Williamson said he stood by his statement.
"There's clear evidence that the methamphetamine market is now smaller than it once was and the efforts of the Government are having an impact."
There is evidence the number of methamphetamine users has halved from 2.2 per cent of the population to 0.9 per cent in the five years to 2012, says a Ministry of Health survey.
But a classified police report obtained by the Weekend Herald estimates that 75 per cent of all methamphetamine is now consumed by 11 per cent of "heavy" users.